As is known in the art, there has been abundant research aiming at ensuring a reasonable quality of video experience for wireless users.
As is also known, the task of providing video streaming of variable quality to a heterogeneous set of receivers with different subscription levels is still an open issue. One challenge is to serve wireless users with video streams that: (i) are of different quality, depending on subscription level; and (ii) provide security guarantees to ensure that only authorized users will access the protected video streams.
In order to illustrate this problem, one can consider the scenario illustrated in FIG. 1, in which nodes A, B and C are interested in a video stream served by node S, but they have paid for different video qualities, for example different layers of a multi-resolution or multi-description video stream. Node S can connect to the receivers through three relay nodes R1, R2, R3 in wireless range, but with poor channel quality. Due at least in part to the noisy nature of the wireless medium, at least some packets transmitted by node S are lost. Reliable video transmission, however, requires node S to retransmit the lost packets using feedback received from nodes A, B and C. Moreover, relays R1, R2, R3 need to synchronize and schedule transmissions to ensure that each node A, B, C receives all packets without duplicates. Under this scenario, video quality can decrease, because some video frames are not delivered in a timely fashion and are therefore skipped.
Moreover, given the broadcast property of the wireless medium, nodes that did not have subscription access to certain layers can potentially overhear the transmitted packets. In FIG. 1, for example, node B could overhear layer 3 frames. Preventing unauthorized access to certain layers in the presence of relay nodes thus imposes a challenging security problem, in particular because encryption of the complete video stream is often deemed unfeasible in resource-limited mobile terminals.
Furthermore, real time decoding of high-quality video already consumes a great deal of processing power, and can become overwhelming in conjunction with resources required for the decryption of large files. Moreover, a lossy wireless medium imposes additional requirements to the security mechanisms, such as robustness to losses and limited synchronization to prevent scheduling problems.
To reduce the amount of processing power required, one can reduce the complexity of the decoding by partially encrypting the video data. However, it is relatively difficult to evaluate the degree of security provided by partial encrypting schemes. The use of layered coding in wireless scenarios was seen as promising, but it is likely to yield prioritization and scheduling problems. For instance, some prior art work has shown that even a relatively simple prioritization of a base layer is not a trivial task.